The present invention relates generally to the field of magnetic field detectors and devices, and more specifically to current monitoring devices or current leakage detection devices, including such detectors.
In accordance with the present invention, there is provided a magnetic field detector, comprising a sensor exhibiting an impedance that changes as a function of field strength at the sensor, the sensor comprising at least one wire arranged in a closed path, or a substantially closed path, which in use encircles at least one wire carrying a current, and a measurement device responsive to the impedance of the sensor, characterised in that the sensor is positioned on at least one magnetic element which forms a closed path, wherein said sensor is substantially insensitive to the position of the or each wire with respect to the sensor.
Preferably the sensor comprises material exhibiting giant magneto-impedance or asymmetrical giant magneto-impedance. Magneto-Impedance (MI) is an effect where the presence of a magnetic field causes the impedance of a material to vary. In this context impedance includes both the in-phase (resistive) component and the out of phase (reactive) component of impedance. A suitable sensor can be formed, for example, of cobalt iron silicon boron wire.
By providing a magnetic field detector in which the sensor is placed adjacent a magnetisable material or a material exhibiting relatively high magnetic permeability that forms a closed loop around one or more current carrying conductors, much improved sensor performance is achieved.
Consider the case of an infinitely long current carrying conductor. It is well known that the magnetic field falls of with distance r, as 1/r.
In testing, the inventors had noticed that movement of the current carrying wire(s) away from the axis of the sensor, when the sensor has a circular loop of magnetic wire, could cause variations in the measurements provided by the sensor. These systematic shifts due to changes in geometry need not be a problem if the geometry is, in use, known and static.
However, the magnetic ring of material removes this sensitivity to geometric offset. It is believed that this is because the flux density within the material of the ring must be uniform and, from application of Amperes law ∫B·dl=μ0I. Thus the positioning of the wire passing through the loop becomes unimportant.
Advantageously two magnetic elements are provided with the sensor placed therebetween. In this configuration the magnetic material acts to shield the sensor from external or environmental fields whilst also making the sensor less sensitive to the positioning of the current carrying conductors.